The present invention relates generally to an expandable seal assembly for releasably sealing an annulus between the exterior of a cylindrical object and a bore of a surrounding casing or wall, and more particularly, to an expandable seal assembly having a seal element and an anti-extrusion backup in contact with the surrounding casing to prevent seal element blowout at the casing or wall.
As is explained in U.S. Pat. No. 5,311,938, hereby incorporated by reference, in the course of treating and preparing subterranean wells for production, a well packer is run into the well on a work string or a production tubing. As should be understood, the packer supports production tubing and other completion equipment such as a screen adjacent to a producing formation, and also seals the annulus between the outside of the production tubing and the inside of a well casing or wall to block movement of fluids through the annulus past the packer location. The packer carries an annular seal assembly which includes a seal element that is radially expanded or "set" into sealing engagement against the bore of the well casing in response to an axial compression force either hydraulically or mechanically exerted by the packer.
A seal assembly that has been set and sealed against the well casing bore must maintain sealing engagement even at high temperatures and high pressures on the order of 325 degrees Fahrenheit and 10,000 PSI, respectively. Such conditions are typical in deep wells, and may also be experienced from mechanical and/or hydraulic pressures applied externally or internally from the formation and/or manipulation of the tubing string and service tools. Moreover, the seal assembly should be able to withstand variations of externally applied pressures at levels up to as much as 10,000 psi in both directions, and still be retrievable after exposure for long periods, for example, from ten to fifteen years or more.
It is known in the prior art to use a retrievable seal assembly that includes a seal element and metal backup rings or shoes that engage the casing wall to prevent seal element extrusion and/or blowout at the casing wall. However, and as should be understood, such engaging metal backup shoes create a drag during the setting and releasing of the seal assembly, and such drag requires the exertion of considerable additional force during such setting and releasing. Moreover, if the seal assembly is improperly set or if the metal backup rings become disengaged from the seal assembly during setting, a leakage passage may develop across the seal assembly and the seal element in the seal assembly may experience extrusion and/or blowout at the casing wall. Overall, such a prior art seal assembly and all other prior art seal assemblies have been found to work poorly.
A need exists, then, for a retrievable seal assembly having an anti-extrusion backup that does not create a drag during the setting and releasing of the seal assembly, that prevents seal element extrusion and/or blowout at the casing wall and the resultant leakage, that maintains a reliable seal between the tubing string and the well casing, and that performs satisfactorily overall.